Diversity and distribution of nearshore barnacle cyprids in southern California through the 2015–16 El Niño

Abundance, species diversity, and horizontal distributions of barnacle cyprids offshore of La Jolla, southern California were described from May 2014 to August 2016 to determine how the nearshore barnacle larval assemblage changed before, during, and after the 2015–16 El Niño. The entire water column was sampled at five stations located within one km of shore with water depths of 4, 6, 8, 10, and 12 m during 33 cruises that encompassed the time when El Niño conditions impacted the area. Nearshore temperature and thermal stratification was concurrently measured using a CTD. Six identified cyprid species, including Chthamalus fissus, Pollicipes polymerus, Megabalanus rosa, Tetraclita rubescens, Balanus glandula, and B. trigonus, along with four unknown species, were collected in our samples. DNA barcoding was used to confirm identifications in a subset of the larvae. C. fissus was more than eight times more abundant than any other species, and while abundance varied by species, cyprid density was highest for all species except for M. rosa before and after the El Niño event, and lower during the environmental disturbance. There were significant differences in cross-shore distributions among cyprid species, with some located farther offshore than others, along with variability in cross-shore distributions by season. C. fissus cyprids were closest to shore during spring-summer cruises when waters were the most thermally stratified, which supports previous findings that C. fissus cyprids are constrained nearshore when thermal stratification is high. Relative species proportions varied throughout the study, but there was no obvious change in species assemblage or richness associated with El Niño. We speculate that barnacle cyprid species diversity did not increase at our study site during the 2015–16 El Niño, as it has in other areas during previous El Niño Southern Oscillation events, due to the lack of anomalous northward flow throughout the 2015–16 event.